Surround Sound Mastering Limiter and Expander

The threshold of the expander can be adjusted between -50dB and 0dB, and the threshold of the surround channels can be offset by up to ±20dB. The ATTACK ranges from 100µs to 3ms, and the RELEASE has a range of control from 0.1 to 5 seconds. The ratio can be regulated between 1:1 and 1:4. The SHIFT control for the surround channels makes a variation between the factors for 0 and 2 possible. The interconnection ratio between the front and rear channels is adjusted by the CPLE control between 100 % F+C and 100 % F>C. In the central position, the front and surround channels operate independently of each other. Two 10 element, LED chain display systems, display the gain reduction for the front and surround channels separately.

The Integration of the LFE Channel

The LFE channels is used for the transmission of Low Frequency Effects and at best for the addional transmission of a subbass signal. Hence its importance for dynamic processing as well as for the location of signals is practically not existing. Though it has to be considered that the reality of a lot of speaker systems use the subwoofer principle for the simple reason that it results in lower cost and smaller speaker housings. The economical advantage of this principle is obvious, but the disadvantage, that the channel assignment of the 5.1 principle is turned inside out has to be considered. The subwoofer receives not only the LFE signal but also the portions of the other channels which are below about 300 Hz. This can cause considerable compatibility problems which are described in detail with the Dimensions surround direction mixer.

It is not meaningfull to include the LFE channel into the production of the control voltages. This could lead to the low frequency regulating activity of the processors in a disturbing way. Therefore we have added a separate limiter path for the LFE channel. This uses the main level, attack and release parameters, but allows the limiter to process the LFE channel separately from the other channels. This feature is controlled by the LFE LIM switch. Also the standard in all 3 processing sections of the unit is that the processing of LFE channel is regulated by the front channels. This regulation automatically ensures that level conditions of the LFE channel are adapted to and directly relates to the front channels. But since this operational relationship cannot be standardized, and is for the most part program dependent, we have included an option that removes the LFE channel from each processor individually with the LFE OFF switch.

The Principles of Multichannel Regulation Amplifiers

Any regulation amplifier with more than one channels needs to be able to couple the regulation. The easiest way to understand the problem is to have a look at the operation of a stereo compressor. If instead of a control coupled stereo processor, two mono compressors are used, its own momentary level separately controls each channel. Therefore the gain of the stereo signal is never exactly matching. As the location of the program essentially depends on the relation of the left and right levels, the position of the sound sources changes permanently. In any case it is necessary, to regulate left and right synchronously to avoid any changes of the left/right level relation. If only one signal is used for the control, only the signal of this channel will be included. A peak in the right will not cause a regulation, if only the left channel is used for the control. It is therefore necessary to combine both of the channels to form a precise control signal. This has to be done in a way, that the channel with the highest level determines the regulation. Do implement this principle separate control voltage converters are needed. If the channels are mixed and processed by only one converter, the regulation depends on the phase shift between both of the channels. A widened stereo signal will produce a lower control voltage as a coherent mono signal with identical levels and phase positions.

The same principle is needed for the expander, even if this seems to be not true at first blush. If there is no signal in one of the channels, of course, the other channel with signal has to lead the regulation and prevent the expander from fading down.

The Basics of the Compressor, Limiter and Expander

Three separate elements are needed for the dynamic processing of a final mix. The mastering compressor should condense the signal and balance out dynamic imbalances. It needs to have wide control ranges to offer all possible ways of processing. This is not only a required for the threshold and ratio but above all for the influence of the attack and release process. The tonal performance characteristics of any compressor are mainly determined by the adjustment of the attack time. Furthermore, an Autogain function is helpful, as the user is excused from the need to readjust the level permanently. As the level shifts with any changing of a setting, actually the real effect of the compression is blurred very fast. It is very comfortable to have a circuitry that readjusts the output level constant as far as possible.

Eventually, any compression effect is based on the effect, that the level of the program material modifies a basic gain; it just turns out in the way that without an input signal there is the maximum gain. Even with settings with only little punch through, this results in a considerable deterioration of the signal to noise ratio. To compensate these effects it is necessary to have an exactly adjustable expander at disposal, which is able to regulate down the basic gain with small input levels and to fade out further distances. Also with the expander it is important to have full control about all the parameters to suit the operation in the way that there are no unpleasant side effects.

As the compression is supposed to condense the program and, more likely a long attack time is needed to produce positive tonal effects, the compressor is not suited to limit the maximum output level. Admittedly, a lot of mastering compressors have a sufficient range to operate as a fast limiter, but of course, than the use of the normal compressor is not possible at the same time. Therefore, an additional, separate limiter is necessary for a complete dynamic processor. The limiter has to have a very fast attack time, to be able to suppress peaks effectively. The most important point of a limiter is not it's tonal behaviour but its ability to rule out peaks in the range of several dBs without negative, audible effects. Since the compressor changes of course the peak level the limiter has to be equipped with its own regulation circuitry, which evaluates the output signal of the compressor.